This invention relates to a flexure hinge assembly, and, more particularly, to such an assembly for connecting two rotary members while permitting universal movement of one with respect to the other through small angles.
A typical flexure hinge assembly of the prior art is disclosed in U.S. Pat. No. 3,811,172. Other prior art flexure hinge assemblies are referred to and discussed in the last section of this specification, entitled "Discussion of the Prior Art". In the device disclosed in U.S. Pat. No. 3,811,172 and in all but one of the devices disclosed in other patents, an inner tubular member and an outer tubular member are each divided into two end portions and an intermediate portion by pairs of diametrically opposed flexure blades formed in the wall of each member. The flexure hinge assembly thus formed has two pairs of diametrically oppositely located flexure hinges which permit universal movement through small angles between a central gimbal element portion and one end portion thereof and has two pairs of oppositely disposed flexure hinges which permit universal movement through small angles between the central portion of the gimbal element and the other end portion of the flexure hinge assembly.
Each aperture in each pair of circumferentially adjacent apertures is connected by a slot to a circumferentially spaced aperture not in the pair of apertures.
The slot-aperture pattern subdivides each tubular member into at least two portions interconnected by the blades exlusively.
Any one of the portions between a slot-apertures series and one of the circular edges of the tubular member or another such series is - or forms a part of - a gimbal, driven or driving element. cutouts around the tubular members separate them into substantially ring-shaped elements, except for the flexure blades.
Each flexure hinge comprises a pair of flexure blades with mutually orthogonal planes of flexure. One flexure blade of each pair in a flexure hinge is formed by a pair of circumferentially adjacent apertures in the wall of the inner tubular member, the other being formed by a pair of circumferentially adjacent apertures in the outer tubular member. A flexure blade may be used where required angular motion about a compliant axis is limited and zero friction and backlash are of prime importance. A flexure blade is a mechanical member which is compliant in bending about one axis but rigid about the cross axes. The term compliance, as used herein, is the reciprocal of the spring rate.
Also, in prior art devices one of the flexure blades in each flexure hinge formed by a pair of flexure blades is oriented to contribute great axial strength along the longitudinal axes of the flexure hinge assembly, and the other blade of the same pair is orthogonally oriented to provide the necessary radial stiffness required for an isoelastic suspension.
However, in the known method of producing an assembly of the prior art, the thin flexure blades in each of the concentric inner and outer tubular members are formed by separately machining four pairs of equiangularly spaced circumferentially adjacent apertures in the walls of one tubular member, and machining four pair of equiangularly spaced, circumferentially adjacent apertures in the walls of the other tubular member to bring them into a coaxial positional relationship. This separate machining of pairs of adjacent apertures to form flexure blades to the degree of accuracy required for use of the flexure hinge assembly in gyroscope instruments is extremely difficult and time consuming. Each radially adjacent pair of flexure blades of which one blade is in the wall of the inner and the other blade is in the outer tubular member, must have a common, colineal flexure axis. The flexure axes of all the flexure hinge assemblies should intersect as accurately as possible, at a common center constituting the pivot point and lying on the longitundinal axis of the flexure hinge assembly. As a result of the prior art method of machining apertures forming flexure blades separately and individually in one and in the other of the tubular members, slight misalignment between the flexure axes of the inner and outer tubular members frequently occurs upon assembly. This leads to the introduction of excessive and unsymmetrical spring rates and other sources of malfunction in the finished flexure hinge assembly. Further, the formation of flexure blades according to the method of the prior art results in flexure blades which have a non-uniform thickness across their respective flexure axes, i.e., bending axes, which results in differences in the spring rates of the flexure blades.
One application for a universal joint for small angle deflections is a free gyroscope. A free gyroscope having a spinning rotor element, when mounted on a supporting structure, operates in such a manner that the supporting structure can be turned or translated without applying disturbing torques to the rotor element. In a practical device, control torques are applied to the rotor element for precessing the rotor element in a controlled manner. A free gyroscope customarily includes angular sensing devices for detecting angular misalignment between the spin axis of its rotor element, frequently called the rotor, and either its shaft axis or a housing-fixed axis. The angular misalignment so detected may then be used, after appropriate application, to apply a torque to a supporting structure, such as a gimbal element of an inertial platform, to cause the supporting structure to follow the rotor spin axis, thereby nulling the angular misalignment. Alternatively, a sensed angular misalignment may be utilized to apply a torque directly to the rotor element in what is known as "caged" or "captured" operation.